This invention relates in general to electro-optic modulators, and more specifically, to devices and methods for modulating a light beam in accordance with varying voltages which has increased sensitivity and is particularly adapted for use with optical fiber systems.
Over the years, systems have come into use which use electrical signals passing through wires from input means to sense information and return the information to a distant location for use. These systems are much lighter in weight, occupy little space and provide redundant wiring paths to protect against loss of contact if one wire is damaged. Unfortunately, these systems are subject to short circuits or other damage to the wires, electromagnetic interference (EMI) from nearby wiring or electrical devices and are potentially subject to destruction by electromagnetic pulses (EMP) from nuclear blasts or other sources. There is a particular need to overcome these problems in military aircraft, missiles and ships and in numerically controlled machine tools and robotics where EMI and EMP pose serious problems.
Recently, considerable interest has developed in using optical fiber systems for passing information rapidly and accurately from a remote sensor over long distances. Optical fiber systems have many advantages over the wired electricalcircuits EMI and EMP, short circuit potential and are lighter in weight which is very important for aerospace applications. Typical fiber optic control systems are disclosed by Sichling in U.S. Pat. No. 4,246,478 and Blackington in U.S. Pat. No. 4,313,226.
Many specialized devices have been used in optical fiber systems for transducing electrical or mechanical position or quantity signals into a proportional light signal suitable for transmission through a fiber. Typical of these are the systems disclosed by Walker in U.S. Pat. No. 4,454,418 and Lockett et al. in U.S. Pat. No. 4,479,264. In many cases it is necessary to transmit light from a friendly environment (e.g., low temperature with no electro magnetic interference such as an aircraft electronics bay) to a hostile environment (e.g., flight controls, engine controls) encode data by modulating the light beam with an electrical signal, and returning the modulated light back to the friendly environment for translation and use. A number of such electro-optic modulators are in use, such as those described by R. A. Becker in his paper, "Broad-Band Guided-Wave Electro-optic Modulators", IEEE Journal of Quantom Electronics, Vol. QE-20, No. 7, July 1984.
In order to obtain the desired light modulation signals in electro optic materials with minimum electrical power it is necessary to use very short, narrow electric field paths together with extended light paths within an electric field and within the electro-optic medium. Present modulators capture light emitted from an optical fiber, pass it through an electro-optic material, recapture the light and direct it into a second fiber for retransmission to the point of original transmission. This is cumbersome and inefficient, requiring dual fibers for each line.
In some modulators, very small electrodes are placed on the electro-optic material surface with very small spacing. This result in a high electric field over a short optical path. In other cases, electrodes are placed on each side of a very thin electro-optic material which results in a longer light path but spacing is still limited by the practical minimum thickness of the electro-optic material, again resulting in undesirably high voltage. Attempts have been made to overcome this problem by cutting shallow parallel grooves in the electro-optic material and placing electrodes in the grooves. Light is then passed through the material perpendicular to the electrodes. This results in closer electrode spacing, because the mechanical integrity of the material is such that the gap between the grooves can be very thin when compared to sheet material. However, the depth of the grooves and thus the light path length is limited by the structural strength of the material.
An electrical bias has been used to reduce the voltage required for optical attenuation. The required voltage is a sine squared function of light attenuation so that the required voltage change for a given amount of light attenuation is less at mid range as compared to voltage onset. I have found that an equivalent effect can be obtained with an optical bias created by circular polarization of the light or by straining the electro optic material to partially pre-rotate its polarization. An equivalent effect can also be created in bifregent electro optic material by "angular turning" the light beam with the materials principal axis.
Electrical bias is sometimes impractical if the modulator power supply is transmitted by light and converted to electric power by photodiode because of limited power and voltage which is practically available.
In addition to problems present electro-optic modulators have with high voltage requirements, electric bias, dual fibers for each information sensing circuit, requiring single mode fiber in lieu of multimode fibers, and undesirable large size and weight, many known modulators also include optics-to-air interfaces which are open to possible fogging or other contamination in a hostile environment which can reduce system efficiency or even render the system inoperative.
Thus, there is a continuing need for improved electro-optic information sensing systems and methods which are more efficient, lighter in weight, have lower voltage and power requirements can operate with multi-mode fibers and without electric bias, and reduce the possibility of interference or contamination by outside materials and forces.